There are thousands of article directories out there. The problem is, which ones do you submit your articles to?
High page rank is a very important factor. While it's not the be all and end all, it certainly means that Google thinks the site is an authoritative one. A bio-box backlink from one of these sites will definitely help quite a bit. That's why I always try to submit to a high page rank article directory first before submitting it to others. Google will most likely index it there first which will mean more link juice. (That is, if you submit to a lower page rank directory, then a high page rank one later, chances are it will index the lower page rank one first, meaning that your backlink will have a bit less value.)
Three of my favourite article directories are Ezine Articles, Article Dashboard and Go Articles. They all have high page rank and heaps of traffic.
Go Articles is the most straightforward. Your article goes live straight away. Article Dashboard usually takes several days before approval. At first, Ezine Articles takes ages. However, once you've had ten articles accepted, things speed up considerably.
From what I can tell so far, Ezine Articles delivers the best SEO benefits. So I suggest that writers submit their article there first, wait for it to be approved, then give it a few days before using the other directories.
Artcles Base is another good article directory. However, the downside is that the bio-box links are nofollow.
There are many, many others worth submitting to, of course. Here are two good lists which include both page rank and Alexa rating.